The silicon-on-insulator (SOI) platform is a promising candidate for future ultra-compact photonic integrated circuits because of its compatibility with complementary metal-oxide semiconductor (CMOS) technology. The high index contrast between the silicon core and the oxide cladding allows for the fabrication of short waveguide bends and therefore circuits with a high degree of integration. Efficient coupling between a high index contrast waveguide and an optical fiber is an important issue, but can be difficult due to the large mismatch in mode size and mode shape between the fundamental mode of the waveguide and the mode of the optical fiber.
Out-of-plane grating couplers may be used as a solution to this problem. Due to the large index contrast such gratings can be very compact and broadband. Furthermore, the out-of-plane approach can make polished facets unnecessary and can enable wafer scale testing of integrated optical circuits. However, this approach has several inconveniences. A first inconvenience is the limited efficiency of coupling between optical fiber and silicon waveguide modes. This can be addressed by incorporating a bottom mirror to redirect the downward refracted light, by applying a poly silicon overlay or by forming slanted slits. A second inconvenience is the need for an adiabatic taper, which typically can be on the order of 500 μm long.
One major inconvenience in the use of out-of-plane coupling is that substantially vertical coupling (i.e. coupling in a direction that is substantially perpendicular to the average plane of the integrated optical circuit) tends to suffer from large second-order Bragg reflection back into the waveguide (e.g., when coupling light between a waveguide and a fiber). Accordingly, off-normal (or off-vertical) coupling is typically used. Off-normal coupling is typically done at about 10° with respect to the normal to the average plane of the integrated photonic circuit. When typical gratings are designed for perfectly vertical or substantially vertical coupling, the second Bragg diffraction order can cause large reflections, resulting in a low coupling efficiency. It has been proposed to use a thin slit adjacent to the grating, with the purpose of achieving destructive interference for the second order Bragg reflection. However, this approach would not be completely satisfactory because reflections are only canceled in a narrow wavelength range.
Light generation in silicon is difficult and therefore there is a need for integrating optical components based on other materials, such as for example optical components comprising III-V semiconductors, on silicon integrated photonic circuits. Examples of such optical components are light sources, such as e.g. Vertical Cavity Surface Emitting Lasers or VCSELs.
A first method for the integration of III-V semiconductor based light sources on silicon integrated circuits is bonding of a III-V semiconductor wafer or parts of such a wafer onto a processed silicon wafer, either by direct bonding or by bonding with an intermediate polymer layer. However, the yield and reliability of these techniques are currently too low for commercial application. A second method is the integration of finished III-V semiconductor devices, such as for example horizontal cavity lasers (e.g. distributed feedback (DFB) lasers or Fabry-Perrot lasers) or vertical cavity lasers (VCSELs) by flip-chip soldering or metal bonding. In this case the coupling of the optical mode from the light source to the underlying device is crucial. In case of horizontal cavity lasers, this coupling can be obtained by etching angled facets on the horizontal cavity lasers and providing the underlying circuit with vertical grating couplers. Horizontal cavity lasers are intrinsically bigger than vertical cavity lasers. As they require more space on the circuit, they are less interesting for high-level miniaturization. Moreover, the etching of angled facets on horizontal cavity lasers is a complicated, expensive and time-consuming process. Vertical cavity lasers do not require angled facets for light coupling to integrated circuits but direct vertical coupling to a waveguide can require a special grating coupler that allows substantially vertical coupling, as for example described by G. Roelkens et al. in “High efficiency grating coupler between silicon-on-insulator waveguides and perfectly vertical optical fibers” in Optics Letters, Vol. 32, No. 11, 2007. However, this type of grating coupler has not yet been experimentally demonstrated and it does not function in a broad wavelength range.
There remains a need for substantially vertical coupling between optical elements and waveguides on photonic integrated circuits over a broad wavelength range. Substantially vertical coupling can facilitate fiber mounting and thus lead to lower packaging costs, especially in case of one- or two-dimensional fiber arrays. Vertical fibers can also be more advantageous for wafer scale testing schemes with multiple fibers. Substantially vertical coupling can further open opportunities for bonding of optical components such as for example III-V light sources on silicon photonic integrated circuits.